Another thing that I do not think has been covered is patterning of the various loads.
Years back a friend of mine was reloading 1650fps in 1 oz and made all sorts of claims. He gave me 6 shells and I patterned them with 2 chokes. It was simply the most awfull patterning I have ever seen. At 40 yds in 30" circle holes the size of a soccer ball
it was less than 20% hit rate and the same choke in a factory 1475 was 80% Basically the extra speed blew the pattern all to heck.
A couple of years latter I was then patterning factory 1600fps VS 1475. Again the higher speed had a poor pattern compared to the 1475.
Now that being said I do remember the 1325 and 1350fps loads of 15+ years ago, and they crippled far more ducks that the 1475 did. It was real frustrating shooting those slower shells of 15-25 years ago. The "speed kills" came out of that era when hunters found that a 1450+ worked a LOT better (killed VS cripple ratio). For me it was night and day difference and when steel shot shells finally got close to lead for effectiveness in shorter ranges (under 40yds). So to use "speed kills" with the 1600 VS a 1475 is to me unproven. The camparison is not linear. Yes in my opinion a 1450-1500 shell of the same payload kill better that a the slower 1300-1350 ones do. But is the 1600 better than than the 1475? I really question that.
So in summation here is my take on it. If too slow, there is not enough foot/lbs of energy hitting the bird. (Foot/lbs is in a single and/or multiple pellets is what kills the bird). And going too fast blows the pattern. So the key is in between, having speed and yet a good pattern. I found the 1450-1500 to be that balance. So I have used 1 1/8 loads for years. Now the disclaimer ~ I have not shot the newer shells for a good 6-7 years, so I have no idea if the patterns have gotten better over time with the factory load over 1500fps. Also back then there were not 1450+ loads available in 3" in 1.25 oz. But if today you can find a 1.25 oz load in the 1450fps range AND it patterns well for you, obviously it would be better than a 1 1/8 oz as it has more pellets for a better chance of a clean kill because there is more foot lbs of energy (EG potentially an axtra pellet hitting the bird).
This has been a good topic gentlemen, I wish you all good hunting to all this fall with what ever you use.
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God, help me be the man that my dog thinks that I am.